FIG. 1 shows a diagram of a typical system for recording ridge patterns. A source of light 1 radiates onto a component 2 which determines the position of the scanning surface 3 for the subject to be recorded, such as the ridge lines on the finger or the palm of the hand. On the scanning surface, the luminous flux from the source of light ends up carrying an image of this ridge pattern on the basis of the differences in the reflection of areas corresponding to the troughs and peaks of the ridge pattern. The optical system, as a rule including a collecting lens 4, a system of mirrors 5, an objective lens 6, protective glass 7 and microlenses 8 over the image sensor, takes this flux and creates an image of the ridge pattern on the light-sensitive surface 9 of a multi-element image sensor. The image sensor converts the image from an optical image into an electronic digital image in the form of an array of intensity values proportional to the radiant flux incident on the corresponding light-sensitive element, and transmits this image to the electronic memory 10. The processing unit 11 standardizes the scale of this electronic image, thus creating the output image of the system.
The component, which determines the position of the subject to be recorded is, as a rule, designed as an optically transparent isosceles rectangular prism. However, there are variants in the design of the system for recording ridge patterns in which prisms of complex form, cylindrical components or plane-parallel plates act as the component determining the position of the scanning surface. In rarer variants, the body element of the system is the component indicating the position of the scanning surface.
The number of mirrors in the optical system may vary and determines the shape and overall dimensions of the system.
The radiation sensor, as a rule, is constructed as a bar or matrix of metal oxide semiconductor transistors or charge-coupled devices.
A common disadvantage of these systems is their considerable size. This is due to the fact that the optical axis preferably extends from the reading surface at an angle close to 45°, and the objective lens for image formation of the required quality must be placed to a considerable distance from the reading surface. With the aim of bending the optical axis in order to reduce the overall dimensions of the system the mirrors are used.
At the same time, the widespread use of biometric identification and the increased requirements for reducing the scan time have recently led to the need for mobile scanners for fingerprints and palmprints with a large reading area whilst having a small thickness providing a comfortable portability.
However, this problem was not solved by using optical scanners, since any configuration of the mirrors did not allow an efficient deflection of the optical axis immediately after leaving the receiving element in a direction parallel to the reading surface, without detriment to the dimensions in a direction perpendicular to the reading surface. So, despite the rather successfully implemented reduction in the system size shown in U.S. Pat. No. 8,320,645 dated Nov. 27, 2012, IPC G06K9/00, it uses three mirrors and the height of the device (in the direction perpendicular to the reading surface) is significant.
There are a few variants for the design of systems for recording ridge patterns which bring about the required resolution and size of the scanning field whilst having relatively low height.
Thus, U.S. Pat. No. 5,859,420 dated Dec. 1, 1999, classified under IPC G01B11/124, discloses a system in which the dimensions of the system for recording ridge pattern are reduced by replacing the receiving prism with a relatively thin ladder-type prism with several output facets, which refer to separate channels for forming parts of the registered object image, which are then combined into an output image.
This system is the closest analogue to the proposed invention. Despite the reduction in size, this layout increases the number of components in the system, which in turn increases the cost, reduces reliability and productivity, and also increases the power consumption of the device. Moreover, with this layout it is unavoidable to have the image distortion areas that correspond to the passage of rays through the boundaries of the prism steps, what is unacceptable in most applications of identification systems.